1. Field of the Invention
The present invention relates to a seat adjuster for a seat used in a vehicle, an airplane or the like which is capable of adjustments of the seat by moving it longitudinally and, more particularly, to an improved seat adjuster in which a movable rail provided in a cushion frame is connected to a fixed rail fixed to a portion of the vehicle by means of a slider in such a manner as to be free to slide relative to the fixed rail.
2. Description of the Prior Art
Conventionally, a seat adjuster of this type has been found satisfactory in realizing the weight reduction of the entire seat, partly because there is no need for providing fixing member to fixedly connect a cushion frame of a cushion seat with a movable rail since both such structures are formed as integral parts of the same member. Also, since loads transmitted via a reclining device from a back frame of a back seat are borne directly by a slide adjustment device, the front and rear frame portions of the cushion frame may have sufficient strength to carry the weight of an occupant. It is possible to construct the cushion frame front and rear portions in the form of a thin plate structure. The conventional seat adjuster of this type permits efficient production of the seat since there is no need for mounting the movable rail to the cushion frame during assembly.
However, although the front and rear frame portions of the cushion frame can be advantageously constructed in the form of a thin plate structure as mentioned above, when the movable rails provided respectively in both side frames are constructed in the form of a thin plate structure, the movable rail (a') of a seat adjuster (A') shown in FIG. 1(B) is decreased in rigidity and thus, when any loads greater than a predetermined load are applied, is deformed together with a slider (a"), so that both side frames (b') and a back frame (c') are incapable of maintaining their respective predetermined forms as shown by two-dot chained lines in FIG. 1(A). Also, as illustrated in FIG. 1(C), the deformed slider (a") may impair the mobility (sliding function) of the movable rail. For this reason, the movable rail must be of a greater in thickness which, of course, is not desireable if further weight reduction is to be achieved. In FIGS. 1(A), (B) and (C), reference character (a'") stands for a fixed rail.